Yahoo Mail for iOS in version 1.5.9 brings folder management as well as the option to open attachments with other apps.

More or less awaited, this update is part of Yahoo Mail strategy to expand its features. If you use this app, henceforth, you can create new folders, rename, or delete folders you no longer need. Open attachments in other installed apps (e.g. PDF files in the Kindle app, if installed), Fix for the sidebar showing up blank for certain users (especially if your device is set to Arabic/Hebrew)

The app is free and available as a universal app (iPhone/iPad/iPod Touch) in the App Store.